I agree with geezer101 that the disconnected brake line probably went to a splitter. I can't tell from your photos, but I bet it is (or was) attached to the booster below the master cylinder.
Can you determine to what the other end of the disconnected brake line is attached? If it connects to a flexible hose that's bolted to the fender well and descends down toward the frame, that's one of two lines to the rear brakes. Either you have no rear brakes or if you do, I bet someone deleted, modified or by-passed the load proportioning valve that's located on the frame above the driver's side of the rear axle. If that's the case you can get by with that, but it's not the safest. If a front hydraulic line failed, you'll have no brakes at all instead of only the rears as designed.